Composite aerospace structures such as tailfins, wings, canards, and other control surfaces are usually hollow and generally have internal spar webs for structural reinforcement. The location and alignment of these spar webs determines the strength and stiffness of the fabricated part as well as its ability to mate with adjoining assemblies. To eliminate problems of adhesion between structural elements and improve the strength of the finished part, it is highly desirable to cure spar webs integrally with the shell of the part to be molded.
Composite structures with integral spar webs were previously molded using uncontrollable inflatable nylon bags and fixed-size hard mandrels. These nylon bags provided good pressure during curing of the laminate but were prone to tearing. When this occurred, the part and mold had to be removed from the autoclave, vacuum rechecked and/or rebagged, and reinserted into the autoclave for cure. This was a time consuming process which sometimes approached the maximum handling time of the pre-preg material. Exceeding this maximum handling time meant that the pre-preg might not bond and cure properly, and that the part would have to be discarded.
The fixed-size mandrels were typically located centrally within each of the internal cavities of the mold. The mandrels were sized approximately 0.250 inch per side smaller than the internal dimensions of the final part size to allow for the lay-up of the part's laminates.
Because of their undersize condition, fixed-size mandrels were deficient in controlling the positions of the critical spar webs and spar extensions. Spar extensions are continuations of the internal spars which project beyond the skin of the part and are used to fasten the part to the fuselage.
As a result of the mandrels (100) being smaller than the part size, the spar webs were able to move in the space between adjacent mandrels during the forming process (See FIG. 1). Cured spar webs exhibited an unacceptable amount of waviness (101) and were not accurately positioned at their ideal locations. The waviness caused a reduction in the physical properties of the cured composite structure and the mislocation of spar extensions (102) caused interface problems in the next assembly operation of the part. If the mislocation tolerance was exceeded, considerable rework was required, and possibly a costly major assembly could be scrapped.
U.S. Pat. No. 3,962,506 describes a flexible and inflatable mandrel for producing hollow products having non-circular cross-sections. Although commonly used in composites molding, uncontrolled inflatable apparatus do not provide for the accurate alignment and control of the spar webs required in our application.